The ptarmigan moults three times in the year. Its summer plumage is
rather grouselike above, with a good deal of rufous brown; the back
becomes much more grey in autumn; almost all the feathers of the winter
plumage are white. That is to say, they develop without any pigment and
with numerous gas-bubbles in their cells. Now there can be no doubt that
this white winter plumage makes the ptarmigan very inconspicuous amidst
the snow. Sometimes one comes within a few feet of the crouching bird
without seeing it, and this garment of invisibility may save it from the
hungry eyes of golden eagles.
Similarly the brown stoat becomes the white ermine, mainly by the
growth, of a new suit of white fur, and the same is true of the mountain
hare. The ermine is all white except the black tip of its tail; the
mountain hare in its winter dress is all white save the black tips of
its ears. In some cases, especially in the mountain hare, it seems that
individual hairs may turn white, by a loss of pigment, as may occur in
man. According to Metchnikoff, the wandering amoeboid cells of the
body, called phagocytes, may creep up into the hairs and come back again
with microscopic burdens of pigment. The place of the pigment is taken
by gas-bubbles, and that is what causes the whiteness. In no animals is
there any white _pigment_; the white _colour_ is like that of snow or
foam, it is due to the complete reflection of the light from innumerable
minute surfaces of crystals or bubbles.
[Illustration: _Photo: W. S. Berridge, F.Z.S._
BANDED KRAIT: A VERY POISONOUS SNAKE WITH ALTERNATING YELLOW AND DARK
BANDS
It is very conspicuous and may serve as an illustration of warning
coloration. Perhaps, that is to say, its striking coloration serves as
an advertisement, impressing other creatures with the fact that the
Banded Krait should be left alone. It is very unprofitable for a snake
to waste its venom on creatures it does not want.]
[Illustration: _Photos: W. S. Berridge, F.Z.S._